In variable air volume (VAV) heating, ventilation and air conditioning (HVAC) systems, air flow through a duct system is varied to provide thermal comfort and proper ventilation. Multi-point, center-averaging sensors are typically provided at terminal box inlets in VAV systems to measure total and static pressure produced by air flowing through the duct system. The difference between these two measurements, known as “differential pressure,” is proportional to air flow through the duct system. Total and static pressure measurements, which are preferably averaged and amplified by the sensor, are transmitted to a controller that responds by increasing or decreasing the rate of air flow through the duct by opening or closing a damper located downstream from the sensor.
Typical examples of multi-point, center-averaging airflow sensors are disclosed in U.S. Pat. No. 4,453,419 (Engelke) and U.S. Pat. No. 5,481,925 (Woodbury). The sensor disclosed by Engelke comprises a number of pairs of tubes extending radially outwardly from a central hub. One tube of each pair has upstream facing holes and transmits a total pressure signal to a first chamber in the central hub, while the other tube of each pair has downstream facing holes and transmits a static pressure signal to a second chamber in the central hub. The total pressure signals and static pressure signals are averaged and amplified in respective chambers of the hub and are then transmitted to a controller. The Woodbury sensor is similar to that of Engelke except the total and static pressure sensing tubes are not paired and only a single static pressure port is provided at the radially outermost end of each static pressure sensing tube.
Most known airflow sensors operate on a principle similar to that of a standard Pivot tube, traversing the air stream and measuring the pressure at a number of locations in the duct. Such sensors are capable of generating reasonably accurate pressure measurements when the airflow pattern through the duct is substantially symmetrical, that is, when the velocity of air flowing through the duct exhibits a parabolic distribution across the duct section, with the maximum velocity at the center of the duct and the minimum at the walls of the duct.
While symmetrical airflow conditions may exist in relatively long, straight portions of the duct system, the presence of obstructions such as elbows, transitions, flexible ducts and dampers in the duct system cause turbulence that disrupt the symmetrical airflow pattern, and can have a negative impact on the accuracy of the pressure measurements generated by the duct.
Another problem associated with known airflow sensors is that the sensor itself can be responsible for a certain amount of turbulence in the duct system. Sensor-generated turbulence causes a drop in pressure across the sensor, as well as noise in the duct system.
Yet another problem associated with known airflow sensors relates to durability. Airflow sensors are typically supplied by manufacturers as assemblies comprising a sensor installed in either a short sheet-metal sleeve or a terminal box to be incorporated into the duct system. When such assemblies are manually moved from place to place on a job site, it is not uncommon for the assembly to be lifted and carried by grasping one of the arms of the sensor. Since sensor tubes are typically of small diameter (to reduce sensor surface area and minimize flow disruption), they are easily damaged by such handling, the result being that the measurements generated by the sensor may be inaccurate.
Advances have recently been made in the prior art towards overcoming these problems.
Of note in this regard is U.S. Pat. No. 6,237,426 (Gryc et al.). Gryc et al. discloses that under asymmetric air flow conditions, total and static pressure measurements made close to the walls of the duct are poor indicators of actual air flow within the duct. Thus, in the sensor Gryc et al., static and total pressure ports are located a sufficient distance from the walls of the duct so as to generate pressure measurements of acceptable accuracy even under asymmetric flow conditions. Gryc et al. also explains that damper back pressure results in static pressure measurement inaccuracy.
To avoid the problem of static pressure measurements being affected by damper back pressure, static pressure ports are provided on a side surface of the sensor. Gryc et al. also make advances in terms of durability through the use of metal braces. Further, the Gryc et al. sensor provides for signal amplification without increasing face area, unlike Engelke and Woodbury, by experimentally-determined optimum location of total pressure ports.
Despite the advances represented by the Gryc et al. sensor, there is an associated increased manufacturing cost. Thus, improvement to the technology remains of interest.